MOUTH-PARTS, WINGS, AND LEGS 47 



On the whole, therefore, it seems probable that all existing 

 insects descended from winged ancestors, with the pos- 

 sible exception of the silver-fishes and their allies. As 

 none of the latter insects has even traces of wings, some 

 authorities believe that they represent a primitive, wing- 

 less stock ; but, in the opinion of Sir Ray Lankester, 

 their independence of a winged ancestry has been attri- 

 buted without sufficient reason. 



The origin of insects' wings is still open to question. 

 The suggestion has been made that they were derived 

 from gills possessed by remote aquatic progenitors, and 

 some colour is lent to this theory by the fact that the 

 wings are traversed by air-tubes, with which blood spaces 

 are always associated in early stages of development; 

 while there is reason to think that, even in some adult 

 insects, the blood circulates in the wings to some extent. 

 Such an assumption, however, presupposes for insects an 

 aquatic ancestry ; and this is highly improbable. " The 

 immense majority of insects " (writes Professor Carpenter) 

 " are terrestrial or aerial, and the aquatic forms appear to 

 have been modified from their land relations. Such evi- 

 dence is admitted by zoologists as conclusively showing 

 the native element of any class of animals ; mammals are 

 universally regarded as primarily terrestrial, though seals 

 and whales are marine; crustaceans as aquatic, though 

 some crabs and wood-lice live on land. It may be ad- 

 mitted readily that life began in the water, and that to 

 the waters we must go for the remote progenitors of 

 insects. But the class as we know it now is composed 

 of typically land-animals, and we have every reason to 

 believe that its immediate ancestors were air-breathers." 

 These considerations lead us to the more plausible sug- 

 gestion that the structures which have been gradually 

 specialised into wings arose, in the first instance, as para- 



